Let’s Hear it for Our Hominid Ancestors

New Study into Fossilised Ear Bones Provides Information on the Evolution of Hearing in Early Hominids

The three bones that comprise the auditory ossicles (the bones of the middle ear that transmit sound vibrations to the cochlea), may the the smallest bones in the human body but they are having a big impact on the way in which anthropologists view the evolution of hearing in early hominids.

The term ossicles, means tiny bones, and this term is used to describe the malleus (hammer), incus (anvil) and the stapes (stirrup) bones, the three bones that make up the auditory ossicles, although in anatomy and other sciences such as palaeontology, ossicles can refer to any small bone. A team of international researchers led by scientists from the Department of Anthropology at Binghamton University (New York, United States), have been able to compare and contrast the fossilised middle ear bones of a Paranthropus robustus to other fossil specimens such as additional ear ossicles from Australopithecus africanus. The team’s findings suggest that the malleus (hammer) may not have changed much over millions of years but anatomical differences in the specimens of the other ear bones, the incus and the stapes, in conjunction with other morphological differences suggest that we modern humans, hear very differently than our ancient hominid cousins. The research team conclude that modern humans (H. sapiens) have a different range of auditory capacities than these early hominid taxa.

The Oldest Hominid Auditory Ossicles in the Fossil Record to Date

Shedding light on the evolution of human hearing.

Picture Credit: Texas A&M

The picture above shows the three middle ear bones of P. robustus, the stapes, the incus and the malleus.

The Anthropology Department at Binghamton University houses an extensive range of fossil casts of early hominids. However, fossils of the auditory ossicles are only rarely preserved, their size and delicate nature often precludes them from any fossil preservation process.

The American team, assisted by colleagues from Italy and Spain examined a range of middle ear fossils, including a complete ossicular chain (malleus, incus and stapes) from a single individual specimen of Paranthropus robustus. The fossils used in this study, including those from the P. robustus were discovered in the cave sites of Swartkrans and Sterkfontein in South Africa. The dolomitic limestone that dominates the landscape of these areas is dotted with a large number of caves and many fossils of Late Pliocene hominids have been found. These locations have been afforded UNESCO World Heritage Site status, such is their importance to palaeoanthropology.

Binghamton University anthropologist Rolf Quam and his colleagues were very keen to examine the middle ear bones unlike the vast majority of the other 203 bones that make up a H. sapiens skeleton the auditory ossicles are fully formed at birth. Babies and infants have technically more bones in their bodies than adults. As we grow so these bones fuse together to give the 206 bones that are found in most human skeletons. This means that any hominid fossilised auditory ossicles found would preserve the shape that they have always had, there would be no distortion in the data obtained from any study due to making allowances for fusing and growing of the bones when the individual was alive. As the international team of researchers stated, the unchanging size and shape of the auditory ossicles suggest that these bones could harbour a wealth of evolutionary data, perhaps even providing an aid to scientists as they attempt to define more accurately our own evolutionary relationships with other hominids.

Comparing the Auditory Ossicles of Early Hominids

Comparing the malleus of P. robustus (top) to A. africanus (bottom)

Picture Credit: R. Quam/PNAS

The research team noticed that the malleus (hammer) bone in the ancient hominids studied is very human-like. It is very different in size and shape when it is compared to the malleus of extant apes such as gorillas. The skull, teeth and jawbone of P. robustus for example, may be very ape-like, but the malleus is very similar to our own. As Paranthropus robustus is believed to have represented a hominid sub-branch that did not lead directly to more advanced hominids such as Homo ergaster, Homo neanderthalensis and ultimately our own species, this research suggests that the shape and size of the malleus must have a very deep phylogenetic origin. In other words, this trait could be traced back to a common ape/human ancestor deep into the Pliocene Epoch.

The incus (anvil) and the stapes (stirrup) in contrast, are much more ape-like. This means that the auditory ossicles of ancient hominids show a combination of human and ape characteristics. The research paper, published in the prestigious journal “The Proceedings of the National Academy of Sciences”, leads to an intriguing conclusion. If the shape of the auditory ossicles of ancient hominids were different from our own, then their hearing abilities are likely to have been different too. This study lends support to the theory that these ancient hominids were not capable of producing speech as we do. Certainly, they could communicate but based on the shape of their ear bones, these ancient human-like creatures may have struggled to pick up the range of acoustic frequencies that we use in everyday conversation.

Chimpanzees struggle to pick up the frequency of human speech, the evolution of the incus and the stapes could perhaps provide evidence of the development of human communication within the fossil record. However, the paucity of the known fossil material rather excludes this possibility for the moment.

Commenting on the research, Assistant Professor Rolf Quam stated:

“Bipedalism [walking on two feet] and a reduction in the size of the canine teeth have long been held up as the “hallmark of humanity” since they seem to be present in the earliest human fossils recovered to date. Our study suggests that the list may need to be updated to include changes in the malleus as well. More fossils from even earlier time periods are needed to corroborate this assertion.”